Cyclic stressing for suppression of strain aging



Nov. 3, 1970 A. w. KLISOWSKI CYCLIC STRESSING FOR SUPPRESSION OF STRAIN AGING Filed April 17, 1967 ANNEALED CYCLED, AND AGED 26 DAYS RI.

5 ll STRAIN m 2 82 E wSEm 5 STRAIN m 2" Fl (5. l

YIELD POINT gELONGATION ANNEALED FIG. 3

YIELD POINT ELONGATION TEMPER ROLLED AND TEMPER ROLLED AND AGED 90 DAYS R.T. THEN CYCLED AND AGEDJ l4 DAYS RT.

% STRAIN m 2'' LROLL REDUCTION cvcuzo AND AGED TEMPER 89 z mwmEb TEMPER ROLL REDUCTION 2 AGED I5 min. 2|2F I 009 z wmwEw 5 STRAIN m 2' A s STRAIN m 2" INVENTOR ADAM W. KLISOWSKI ATTORNEYS United States Patent 3,537,913 CYCLIC STRESSING FOR SUPPRESSION 0F STRAIN AGING Adam W. Klisowski, Wintersville, Ohio, assignor to National Steel Corporation, a corporation of Delaware Filed Apr. 17, 1967, Ser. No. 631,438 Int. Cl. C21d 1/04, 7/00 U.S. Cl. 1484 8 Claims ABSTRACT OF THE DISCLOSURE Improved fiat rolled mild steel is produced by cyclical stressing below the endurance limit of the steel to mask yield point elongation and remove strain-aging characteristics. Stress ranges and number of cycles for steel in annealed condition, temper rolled and aged steel, and nonaged temper rolled steel are prescribed.

This invention is concerned with modifying the yield point elongation and improving the strain-aging characteristics of flat rolled mild steel. More particularly, this invention is concerned with mechanically suppressing the yield point elongation of fiat rolled mild steel without substantial strain hardening of the steel.

Detrimental effects of strain-aging of mild steel include an increase in hardness and a decrease in ductility These detrimental effects manifest themselves in breakage of deep drawn parts and appearance of stretcher strains known as Luders Lines in mildly drawn parts.

Strain-aging characteristics can be eliminated by stabilizing the steel. An example is fully deoxidized (killed) steel in which the strain-aging effect is not manifested, for practical purposes, because dissolved oxygen and nitrogen are at a minimum. In addition to the increased cost of killed steel, the rimming action required during casting to produce rimmed steel is not available with killed steel. Since rimmed steel has desirable features not available with killed steel, e.g., better surface finish with less surface defects, more responsiveness to heat treatment, lower cost, and the like, tremendous efforts have been made by steel producers and steel users alike over the years in an attempt to remove the effects of strainaging and yield point elongation in rimmed steel.

One quantitative measure of strain-aging characteristics is yield point elongation. This is a part of the total elongation of the steel occurring after the yield point stress is reached at which the steel deforms very rapidly under a constant or decreasing stress (load).

One effective, though temporary, method of masking yield point elongation in nonstabilized mild steel is to temper roll the steel after annealing. Temper rolling, with a reduction in excess of 1%, usually around 2 to 2 /2%, will smooth out the yield point elongation region of the stress-strain diagram of the steel so that the steel goes from its proportional range into its permanent set range without marked yield point elongation. The steel is then more suitable for drawing operations.

However, the beneficial effects of temper rolling are short-lived. Detrimental effects of strain-aging can return to an objectionable degree within a week, depending on such factors as the amount of temper roll reduc tion, storage temperature, and the nitrogen content of the steel. As is known to those skilled in the art, strainaging is accelerated at temperatures above room temperature. Therefore, tight scheduling of nonstabilizing steels is of great importance in fabrication of mild steel since, for reasons of economy, deep drawing sheets are stressed so nearly to the breaking point that even relatively slight changes in their properties may cause a sizable increase in breakage loss or stretcher strains, and thus render the material entirely unsuitable.

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Other disadvantages of temper rolling are a decrease in ductility and an increase in hardness in an amount dependent upon the percentage of reduction of the temper roll pass.

An object of the present invention is to modify the yield point elongation, and improve the strain-aging characteristics, of fiat rolled mild steel, without substantial loss in ductility or hardening effect on the steel. In accordance with the invention, yield point elongation is effectively suppressed in annealed product making it more suitable for drawing operations without temper rolling. With other product, temper rolling can be partially or completely supplanted, and strain-aging characteristics of temper rolled product are either eliminated for practical purposes or effectively retarded so as to eliminate scheduling problems.

With fiat rolled mild steel in an annealed condition yield point elongations up to 5% are common. Because of such performance, annealed steel sheet has been unsuited for many normal uses of fiat rolled mild steel in the past. The present invention suppresses or substantially eliminates yield point elongation of annealed steel sheet without the hardening effect of temper rolling.

Considering temper rolled product which has aged, the present invention suppresses or substantially eliminates the yield point elongation of the aged product without the hardening effect of an additional temper roll. This contribution of the invention eliminates serious problems for both steel producers and users when their product has aged during storage by permitting substantial recovery of the original temper rolled properties of the steel without resorting to a mechanically detrimental and uneconomic procedure of the prior art, that is, an additional temper roll.

When producing temper rolled mild steel, the present invention permits a substantial decrease in the temper rolling percentage reduction while effectively masking yield point elongation and eliminates strain-aging for considerably longer than average storage times.

In further description of the invention, reference will be had to the accompanying drawings wherein:

FIG. 1 is a portion of the stress-strain diagram of conventional fiat rolled mild steel in annealed condition,

FIG. 2 is a portion of the stress-strain diagram of fiat rolled mild steel in annealed condition, treated in accordance with the present invention,

FIG. 3 is a portion of the stress-strain diagram of conventional temper rolled and aged mild steel,

FIG. 4 is a portion of the stress-strain diagram of temper rolled and aged mild steel treated in accordance with the teachings of the present invention,

FIG. 5 is a portion of the stress-strain diagram of conventional temper rolled, artificially aged, mild steel, and

FIG. 6 is a portion of the stress-strain diagram of fiat rolled mild steel produced in accordance with the teachings of the present invention.

The invention teaches cyclical reverse flexing of fiat rolled mild steel carried out at a stress level lower than the endurance level of the fiat rolled mild steel. That is, at a stress lower than the maximum stress at which the steel can endure an infinite number of stress cycles. The stress applied can be above the true elastic limit for the steel, that is the maximum stress to which the steel can be subjected without any permanent strain, but should not exceed the proportional limit for the steel. The latter being the maximum stress at which strain remains proportional to stress. This portion of the stress-strain diagram of a flat rolled mild steel can be defined as the microstrain region wherein microscopic stresses, presumably within the grains may occur, but macroscopic stresses are not developed. The cyclical stress taught by the present invention can exist in this region but not above this region in the stress-strain diagram.

sented in Table I below, all specimens are rimmed steel sheet .075 thick:

TABLE I.ANNEALED COMMERCIAL SHEET .075" THICK Cyclic bending Elongation in 2" Max. Yield Tensile stress, strength, strength, Y.P., Uniform, Total, Speelmen p.s.i. Cycles Aging lower, p.s.i. p.s.i. percent percent percent Another important teaching of the present invention is selection of a proper number of stress cycles for a particular product.

Beneficial effects of the present invention on flat rolled mild steel can be determined from the accompanying stress-strain diagram drawings and specimen data tables. All specimens covered in the accompanying drawings and tables are commercial rimmed steel sheet with the following analysis of rim and core metal after rolling:

.0160 N .0036 Si .005

and about .0l% copper, .0l% nickel, .01% chromium, and .016% molybdenum. However the invention is aplicable to any fiat rolled mild steel which has not been fully stabilized and is not limited to steels of the above chemical analysis. Carbon content of mild steels can extend to .15% but under normal conditions carbon is about .06% to .08% when analyzed at the hot metal stage. Ordinarily the phosphorous, sulphur, and silicon levels of mild steel are controlled and attempts are made to keep the oxygen and nitrogen level as low as possible.

FIG. 1 is a portion of a stress-strain diagram of a specimen of conventional flat rolled mild steel in the annealed condition. The yield point elongation of this specimen is 3 /2 The yield point elongation of annealed steel is substantially suppressed in accordance with the present invention by cyclically stressing the steel, for example by cyclical reverse bending. The cyclical stressing is carried out substantially below the endurance limit of the flat rolled mild steel, e.g., not higher than about 80% of its endurance limit. The stress can be varied between about The absence of detrimental effects on the annealed steel from treatment in accordance with the invention should be noted. Substantially no strain hardening occurs because the stressing is maintained below the endurance limit of the steel. The cycled specimens of Table I have been aged for 26 days at room temperature with no change in the yield point elongation occurring. Since the steel cycled is in the annealed condition, that is not temper rolled, no strain-aging is to be expected. Proper selection of the stress and the aggregate number of cycles, within the ranges set forth above, can therefore produce a masking of the yield point elongation equivalent to temper roll ing up to 2 /2% reduction. These benefits are obtained without the problems of aging and Without substantial strain hardening which are experienced with temper rolling.

In treating temper rolled mild steel which has aged, the invention teaches cyclical stressing at about 15% to about 30% of tensile strength. The aggregate number of flexing cycles applied should fall in the range of about 50,000 to about 1,000,000 cycles. Stresses around 15% of tensile strength are preferred.

FIG. 3 shows the yield portion of the stress-strain diagram of a specimen of conventional flat rolled mild steel which has been temper rolled and aged 90 days at room temperature subsequent to temper rolling. Note from FIG. 3, and Table II below, that a yield point elongation of 1 /2% is experienced under these conditions.

The yield portion of the stress-strain diagram of a specimen originally identical to that of FIG. 3, but which has been cycled in accordance with the present invention, is shown in FIG. 4. Note in FIG. 4 and Table II below that the yield point elongation has been suppressed to about /2%.

Table II presents data on the specimens of FIGS. 3 and 4, and data on two additional specimens, all specimens being .075 inch thick rimmed steel sheets.

TABLE II.ANNEALED COMMERCIAL SHEET .075 THICK Cyclic bending Elongation in 2" Max. Yield Tensile stress, strength, strength, Y.P., Uniform, Total, Specimen p.s.i. Cycles Aging lower p.s.i. p.s.i. percent percent percent Fig. 3 90 Da. R.'I. 22, 490 40, 110 1% 29 39 Fig. 4 6, 000 5. 17 X10 20, 600 39, 500 25% 38 3A..- Da. RIP--. 21,290 39,850 1% 28 39 4A... 6, 150 1.04X10 20, 200 39, 250 26% 36 1 90 Da. R.T. Cycled then 14 Da. R.T.

15,000 and 20,000 pounds per square inch for annealed steel, which can be about 40%, but greater than of the tensile strength of the steel. The number of cycles applied to flat rolled mild steel in the annealed condition, can be varied between about one million and 7 million cycles. The preferred stress range is between 17,000 and 19,000 p.s.i. and the preferred cycle range is from one and a. half million to seven million.

The eifects of cyclical stressing of annealed steel in accordance with the present invention are shown in FIG. 2. The yield point elongation of the steel is masked considerably and decreased from about 3 /2 to a value which is not clearly defined but has been designated about 1%. Comparison data of the two specimens shown in FIGS.

It should be noted that the yield point elongation of the temper rolled and aged specimens, treated in accordance with the present invention, is substantially eliminated for practical purposes. Further that the yield point elongation does not return with age since the cycled specimens have been aged at least 14 days after cycling. In addition it should be noted that ductility in terms of elongation is not significantly changed by the cycling whereas a steel user or producer who performs an additional temper roll in order to mask the yield point elognation of aged temper rolled steel will harden the material considerably and cause loss of ductility.

In the production of temper rolled mild steel the present invention substantially decreases the temper roll re- 1 and 2, and data from two additional specimens is preduction required and eliminates strain-aging characteristics for practical purposes. Flat rolled mild steel which is conventionally reduced 2 during temper rolling to mask yield point elongation, need be reduced only 1 to 1 /2%, in accordance with the invention, prior to cycling. The cyclical reverse flexing applied is substantially below the endurance limit of the steel at a stress between about 20% and about 40% of the tensile strength of the steel, with stresses in the range of 20% to 30% of the tensile strength being preferred. The aggregate number of cycles is maintained in a range of about 100,000 to about 2,000,- 000 cycles, with cycles in the range of 100,000 to 500,000 being preferred.

FIG. 5 shows the yield portion of the stress-strain diagram of a flat rolled mild steel specimen with a temper roll reduction of 2 /2%. Aging of the specimen has been accelerated by holding it at 212 F. for minutes. As set forth in Table III below, the yield point elongation of this specimen is about 1% FIG. 6 shows the yield portion of the stress-strain diagram of a flat rolled mild steel specimen which was identical with the starting specimen of FIG. 5. However in place of a 2 /2% reduction temper roll, the specimen of FIG. 6 was reduced only l /2% by temper rolling and was then cyclically flexed. After cycling the specimen of FIG. 6 was aged in an accelerated manner for 15 minutes at 212 F. As brought out by these figures and the data of Table III, the yield point elongation is substantially eliminated.

Table III below includes data on the .080" thick rimmed steel sheet specimens of FIGS. 5 and 6: 30

TABLE lII.-ANNEALED COMMERCIAL' SHEET 6 number of flexing cycles applied is between about 50,000 and about 1,000,000 cycles.

4. The method of claim 1 further including the step of temper rolling the annealed steel with a temper roll reduction between about 1% and about 1 /2% and then applying the cyclical flexing to the temper rolled steel without substantial intermediate aging of the steel, in which the cyclical flexing is applied at a stress above about 10,000 pounds per square inch but not in excess of of the tensile strength of the steel, and the aggregate number of flexing cycles applied is between about 100,000 and about 2,000,000 cycles.

5. The method of claim 1 in which the cyclical flexing is applied by cyclical reverse bending in which the flat rolled mild steel is rolled between vibrating rolls in which the cyclical flexing frequency is in excess of 2,000 cycles per minute.

6. Nonstabilized flat rolled mild steel in annealed condition in which the yield point elongation has been suppressed from a value in excess of about 3% elongation to about 1% or less elongation by cyclically flexing the annealed flat rolled mild steel at a stress level above about 15,000 pounds per square inch but not higher than of the tensile strength of the steel with the aggregate number of flexing cycles being between about 1,000,000 and about 7,000,000 cycles.

7. Nonstabilized flat rolled mild steel which has been annealed, temper rolled with a temper roll thickness reduction of about 2 /2%, strain aged, and then cyclically flexed at a stress level above about 5,000 pounds per .075 THICK Cyclic bending Elongation in 2 Max. Yield Tensile stress, TR, strength, strength, Y.P., Uniform, Tot., Specimen p.s.i. Cycles percent Aging lower p.s.i. p.s.1. percent percent percent Fig. 5 2% 35,640 46, 620 1% 19. 5 31 Fig. 6 11,340 1. 7x10 1% 30,720 710 0 24 39 1 15 min. at 212 F.

Cyclical bending in accordance with the invention 40 can be carried out on reverse bending rolls which are vibrated, can be carried out with electromagnetic vibration, or a combination of these. The rate at which the vibrating is applied will depend on the speed of the line and the number of cycles to be applied, but as a practical manner will ordinarily be in excess of about 2,000 cycles per minute.

The accompanying drawings and tables set forth specific examples of the invention, however the scope of the invention is to be determined from the appended claims.

What is claimed is:

1. Method for suppressing yield point elongation in nonstabilized mild steel which has been cold rolled to desired thickness gage and annealed after cold rolling comprising:

cyclically flexing the flat rolled mild steel at a stress level below about 80% of the endurance limit stress for the flat rolled mild steel with the aggregate number of flexing cycles applied to the annealed flat rolled steel being in excess of about 50,000 cycles.

2. The method of claim 1 in which the annealed flat rolled steel is cyclically flexed at a stress level between about 15,000 and about 20,000 pounds per square inch and the aggregate number of flexing cycles applied is between about 1,000,000 and about 7,000,000 cycles.

3. The method of claim 1 applied to annealed flat rolled mild steel which has been temper rolled after annealing with a temper roll thickness reduction about 2 /2% and allowed to strain age so as to develop a marked yield point elongation in excess of about /z% in which the cyclical flexing is applied to the temper rolled and aged steel at a stress level above about 5,000 pounds per square inch but not higher than about 30% of the tensile strength of the steel, and the aggregate square inch but not higher than about 30% of the tensile strength of the steel with the aggregate number of flexing cycles being between about 50,000 and about 1,000,000 cycles so as to suppress yield point elongation to a value below about /2 elongation.

8. Nonstabilized fiat rolled mild steel which has been annealed after cold rolling, then temper rolled with a reduction below about l /2%, and then cyclically flexed at a stress level above about 10,000 pounds per square inch but not higher than about 40% of the tensile strength of the steel with the aggregate number of cycles being between about 100,000 and about 2,000,000 cycles, the cyclical flexing being applied after temper rolling without substantial intermediate strain-aging of the steel.

References Cited UNITED STATES PATENTS 1,517,354 12/1924 Gillett 1484 2,848,775 8/1958 Ettenreich 148-4 X 3,230,118 1/1966 Tufts 148--l2.4 X 3,247,946 4/ 1966 Klein 1484 X 3,256,119 6/1966 Logan et a1. 14812.9 3,276,918 10/ 1966 Langenecker 148-4 X FOREIGN PATENTS 1,093,064 11/1954 France.

OTHER REFERENCES The Annealing of Low Carbon Steel, The International Symposium on the Annealing of Low Carbon Steel, Case Institute of Technology, Oct. 29 and 30, 1957, pages 24 and 25.

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 1481, 12, 12.9, 36,131 

